Step 1: Get Materials
1.Alarm clock - mine is a cheap one that I got from Walgreens. It gets it's power from the mains and has a 9v battery for backup. I don't remeber how much it cost but it was probably less than $15.
2.Some basic knowledge of electronics and construction techniques. (really?)
Step 2: Get Materials
Step 3: Get Materials
Step 4: More Materials
5.Relay - 12vdc coil, and able to handle your bell supply power.
6.555 Timer - I think pretty much any standard variation will work.
7.2n3903 Transistor - (this may be superfluous)
8.1 M Ohm Resistor
9.Some hookup wire
10.Extension cord - this is optional, but if you use one make sure its the kind with 3 outlets so you can plug in the clock and two wall warts.
11.hardware and some sort of case to put it all in
solder, some sort of support (pcb, breadboard, etc.), soldering iron, multimeter (or just a voltmeter), wire stripper, screw drivers, a knife maybe